1. Field of the Invention
The present invention relates to a dental grinding tool, in particular a dental grinding tool which has a grinding part at the end face, and a method for making the same.
2. Description of the Related Art
In dental care or dental technique, a grinding tool which rotates at high speed by means of an air turbine is used. For this purpose, there is used a grinding tool which is made of steel or stainless steel and has a grinding part formed at the tip part of a cylindrical metal base. Diamond abrasive grains are usually fixed on the grinding part, and have most excellent abrasiveness in the field of grindstones. By driving the diamond grain-equipped grinding part by the air turbine, teeth and dental prostheses, such as crowns consisting of nickel-chromium castings, or the like, can be efficiently ground and processed.
Although the metal base of the grinding tool has usually a common cylindrical shape, the shape of the grinding part is diversified. For example, the grinding part may be cylindrical having the same diameter as that of the metal base, tapered becoming gradually thinner toward a tip part, spherical, etc. Further, the grinding part is classified into some classes by the size of the abrasive grains, such as coarse, intermediate and fine grains.
When a crown is placed on a tooth, a cross-hatched part “d” of the tooth “a” is removed with a grinding tool as shown in FIG. 3. Then, a formation part “c” which serves as a bonded surface with the crown is formed at a boundary part between the tooth “a” and the gum “b”, an impression is taken to form the crown, and cement or the like, is filled between the crown and the tooth “a” for joining. In this joining, no gap is allowed between the formation part “c” and the bonded surface of the crown (not shown). When a gap is left, a larger amount of cement has to be used. This causes a problem that the cement dissolves after joining and the crown may be easily taken off. Therefore, if the formation part “c” is precisely and finely formed and an accurate impression is taken, no gap may be left between the crown and the joining surface, and the crown can be firmly fixed with a small amount of cement. For shaping this formation part “c”, a grinding tool is used in which the end portion of a cylindrical metal base is chamferred into a truncated cone shape and a grinding part is formed by fixing abrasive grains only onto the end face of the metal base.
FIGS. 4A to 4D are diagrams for illustrating a conventional method for making such a grinding tool. First, as shown in FIG. 4A, a tapered chamferred part 1b is formed at the end portion of a cylindrical metal base 1 made of steel or stainless steel. Then, as shown in FIG. 4B, masking is applied on the outer surface other than the end face 1a of the metal base 1. This masking prevents abrasive grains 2 from adhering when the grains 2 are subsequently electrodeposited. The metal base 1 with the thus-applied masking is loaded into an electrodeposition apparatus, and diamond abrasive grains are electrodeposited only on the unmasked end face 1a by the composite plating method. The masking is subsequently removed to obtain a grinding tool in which the abrasive grains 2 are firmly fixed to the end face la, as shown in FIG. 4C.
However, it is difficult to apply masking by leaving only the end face unmasked. The masking is sometimes applied erroneously on the end face or unmasked parts (blanks) are left on the chamferred part. Therefore, by the conventional method, the end face is partly electrodeposited with the abrasive grains or the abrasive grains are electrodeposited on the chamferred part. When such a grinding tool is used, the boundary part between the tooth and the gums can not have a smooth and flat surface because the end face has an area where no abrasive grains are fixed or the abrasive grains fixed to the chamferred part may damage the gums when grinding the boundary part mentioned above. Thus such a grinding tool could not be used and was regarded as a defective product. For such a reason, rejection rate of the grinding tool has reached about 30% in the conventional manufacturing method.
If the abrasive grains were fixed to the end face and the grains were absent on the chamferred part 1b, the product was regarded at least as nondefective. Even in this nondefective product, however, some of the abrasive grains, grains 2′, were protruding over an outer circumferential line 1c demarcating the end face 1a, as shown in FIG. 4C and FIG. 4D. These protruding grains 2′ might often come in contact with the gums and damage the gums. Moreover, the formation part “c” to be created on the tooth by grinding could not be shaped finely. Consequently, it became impossible to obtain an accurate impression. This caused a problem that a gap was formed between the crown and the formation part “c”.
Recently, stereomicroscopes are becoming increasingly popular, and dentists are now capable of confirming the formation part in more detail. Thus such a problem is growing more conspicuous and the solution of the problem is required.